Literally, the narrator is just talking about the academic year starting. The narrator talks about how things had been pretty mellow during the summer session -- it is described as "easy-going." But now things are back to normal.
More figuratively, what's happening here is that the whole mood of the school and of the story has changed. Things were pretty light hearted before Finny's fall and before the war became so important. But now things get a bunch darker. You have more tension, with things like Gene's fight with Quackenbush happening.
So I think the statement has a couple of different levels to it.